Dynamic changes in chromatin structure and function are directly influenced by the post-translational modifications of the DNA-associated histone proteins. These small, evolutionary conserved proteins are targets for a diverse array of modifications, each of which is usually associated with specific DNA-templated processes, such a genome organization and DMA repair. We recently discovered that the PR-Set? histone H4 lysine 20 (H4K20) monomethyltransferase is important in maintaining genomic stability. Based on these findings, my overall hypothesis is that PR-Set7-mediated monomethylation of H4K20 serves a protective function from DMA damage in the mammalian genome and that perturbation of this pathway results in cells prone to oncogenesis. Our recent findings strongly suggest that the protective function of PR-Set? is mediated "indirectly" by its ability to compact chromatin, thereby, sheltering the underlying DNA from damage. In addition, we recently discovered that PR-Set7 interacts with key components of the DNA repair machinery. Based on this, we also hypothesize that the association of PR-Set? with these repair proteins provides an additional layer of "direct" protection from damage. In Aim 1 I will use unique reagents in conjunction with a novel quantitative assay that I developed to determine the role of PR-Set? and monomethylated H4K20 in chromatin condensation and DNA damage at endogenous target loci. In Aim 2 I will use established molecular and biochemical techniques to determine the structural and functional relationships between PR-Set? and the DNA repair proteins. Furthermore, I will determine how perturbation of these interactions can alter the DNA damage and repair response at the endogenous target loci. In Aim 3 I will examine the role of PR-Set? and monomethylated H4K20 in preventing aberrant chromosomal defects, oncogenesis and cancer cell invasion. Collectively, this proposal will illuminate an important fundamental mammalian pathway required for the maintenance of genomic stability that will likely have far-reaching and wide-spread impacts on human health and disease.